As for knowing that Latin was a spoken language, I don't have anything in mind at the moment, but I'm also not an expert on Roman grammarians. However, Aristotle, Plato and other ancient Greek philosophers wrote often about the pronunciation of words. Plato's Cratylus is a specific example that discusses whether sounds have an inherent meaning (a 'correct' meaning) or if they are (in modern terms) arbitrary. Does "dog" mean dog only by convention, or inherently in its sound? (Of course he referred to the ancient Greek words.) Latin grammarians of course made similar comments about pronunciation.

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Also, you are ignoring the counter-evidence I've presented: the descriptions we have about Latin grammar are so inconsistant that it appears to be impossible to make a simple statement such as "Heroes are never forgotten." in Latin.

That isn't evidence of anything. Whether or not I can translate a sentence into German does not prove (or disprove) that German exists. That is entirely irrelevant.

Aside from skepticism, there is no reason whatsoever to reject Latin as having existed or been spoken. There are historical accounts of it. Consider any Roman play that was performed and understood. And from the perspective of linguistics, although the situation is complicated, it is clear that Spanish, French, Italian, Romanian, etc., came from some shared ancestor. (That argument applies to why we know some ancestor of many European languages, which we now refer to as Proto-Indo-European, also existed.)

Some variety of Latin we can refer to as Vulgar Latin existed and was spoken in Rome. Questioning that is just silly. Asking exactly what Vulgar Latin was like is a very good academic question, one that many articles and books have been written about, and a continued topic of interest for future research as well. You can read about it here:https://en.wikipedia.org/wiki/Vulgar_Latin

I'm ready to be done debating this, especially before the conversation devolves further. I've already wasted enough time trying to convince you that water is wet. Believe what you wish.

No inconsistencies:Heroes are never forgotten = numquam obliviscuntur heroes.May heroes never be forgotten= numquam obliviscantur heroes.Children do not learn inflected languages like Latin, Greek, and German by having to study grammar; they learn through listening and composing their own sentences -- which takes a native intelligence. If Latin had never be spoken, there would be no Latin writings. The Romance languages are mostly simplifications of inflected Latin.P.S.Actually, the fact that Egyptian was written does not imply that it ever was a spoken language. But when the writing (like Latin, Etruscan, etc.) is in a Greek-like alphabet, then the writing is actually a record of the sounds of a spoken language. Furthermore, Roman commentators on the diction of some Latin speakers give some information about the history of some "romance" languages. And more..........